The present invention relates to a process for making oligomeric halogenated chain extender compositions and reaction products of such chain extenders, which in turn can be used for making thermally resistant epoxy resin compositions. The thermally resistant epoxy resins, are useful, for example, in electrical laminate applications, such as for the manufacture of printed wiring boards.
There are several commonly used indicators of thermal performance of electrical laminates. One of these is the glass transition temperature (Tg) of the cured resin. Another measure is the thermal decomposition temperature (Td) of the cured resin, which is determined using thermogravimetic analysis (TGA). A third indicator is known as “T260”, which is the time required for a laminate to begin to decompose when heated to 260° C. A similar indicator is “T288”, which measures the decomposition time at 288° C. A fourth, but related, indicator is solder dip resistance, which is the time required for the laminate to begin to delaminate when it is dipped into molten solder at 288° C.
Recently, industry standards have begun to specify that lead-free solders be used to construct electronic devices. The lead-free solders usually melt at higher temperatures than conventional lead-based solders. The use of these solders therefore places greater demands on the thermal stability of the resin phase of the electrical laminate. Conventional resins have not been able to satisfy these added thermal requirements.
Another circumstance that drives the need for better thermal stability is the production of multilayer boards. These are formed by bonding thin pre-processed boards together using prepreg layers. This operation can be repeated several times. With each repetition, the entire board is subjected to a complete thermal cure cycle. As a result, the higher the layer count, the greater is the thermal impact on the inner layer board.
Therefore, it is desirable to provide a resin that can enable the laminate to exhibit the needed thermal properties. Laminates exhibiting a Td of 310° C. or higher are expected to become standard in the industry. The T260 value should be at least 15 minutes, and preferably at least 30 minutes, but values of an hour or more are especially desired. T288 values in excess of 5 minutes are also desired. The Tg should be 130° C. or more, and preferably at least 150° C.
These thermal properties cannot be achieved at the expense of other desirable attributes of the resin and the laminate. The resin must be easily processed, must have acceptable flow characteristics during the lamination step, and must have the necessary physical properties characteristics need to produce dimensionally stable laminates.
Epoxy resins are widely used to make electrical laminates. The resins are often brominated in order to impart the needed thermal properties to them. An example of such a brominated epoxy resin composition is described in U.S. Pat. No. 5,405,931 to Kohno et al. In the process described in that patent, an oligomer having terminal phenolic groups is prepared by reacting an excess of a halogenated phenolic compound with a glycidyl ether of a halogenated phenolic compound. The oligomerization reaction is performed in a melt of the starting materials. This oligomer is advanced with another epoxy resin and then cured to form the polymer phase of an electrical laminate.